Material handling implements such as tractors, bulldozers, and the like, frequently employ multi-speed transmissions which are configured to effect gear ratio selection by selective fluid pressurization of hydraulic clutches. In such a transmission, a plurality of such hydraulic clutches are typically provided, wherein engagement of each clutch by fluid pressurization provides a respective one of the transmission's multiple speeds. Electrically-operated hydraulic valves are typically employed for supplying pressurized fluid to the hydraulic clutches.
Desirably, transmissions of this type are usually arranged to obviate the need for the implement operator to manually engage and disengage a clutch mechanism attendant to transmission shifting, but rather permit the operator to effect selection of the desired transmission speed merely by manipulation of a suitable gear selection control. This control operates through suitable circuitry to operate the electro-hydraulic valves and effect shifting by sequentially pressurizing one of the transmission's hydraulic clutches (for engagement) while relieving fluid pressure in the previously engaged hydraulic clutch. Naturally, efficient operation of the implement is promoted by convenient transmission gear ratio selection.
As will be appreciated by those familiar with the art, it is desirable that a transmission such as described be operated such that shifting of gear ratios is effected as smoothly as possible. This requires relatively precise sequential operation of the transmission's hydraulic clutches. Although fluid pressurization and depressurization of the clutches for shifting is effected quite quickly, operation of the associated electro-hydraulic valves and full clutch engagement or disengagement require distinct and finite time periods.
Accordingly, control arrangements for such transmissions typically function such that fluid pressurization of the hydraulic clutch for a newly-selected speed is commenced prior to relief of fluid pressure in the hydraulic clutch which is in engagement prior to shifting. Operation of the transmission's clutches in this manner is sometimes referred to as "clutch swap", with the time period between initiation of engagement of one of the clutches and disengagement of another of the clutches sometimes referred to as "clutch overlap".
Minimization of "shift shock" or "jerk" is the desired goal in configuring transmission control systems. Shift shock is a function of the time rate of change of acceleration of the implement. As will be recognized, timing of the sequential operation of the hydraulic clutches is directly related to shift shock. If the clutch overlap sequencing time is too short, i.e., the one clutch is disengaged too quickly after initiation of pressurization of the next clutch, both the on-coming and off-going clutches can be momentarily disengaged. When under load, the implement will quickly decelerate; when the on-coming clutch is pressurized and engages, the implement accelerates with a significant shift shock. If the clutch overlap sequencing time is too long, i.e., disengagement of one clutch is not effected quickly enough after pressurization of another clutch is initiated, the implement can jerk forward when shifted, or a momentary clutch "lock-up" in the transmission can occur, again causing a large shift shock.
In the past, control arrangements for such transmissions have been arranged to monitor fluid pressures in the various hydraulic clutches, with clutch sequencing effected in accordance with the changing fluid pressures. While such arrangements can perform satisfactorily, these systems of course do not monitor the actual smoothness of a shift, but instead must be arranged to provide shifting which is a smooth as possible under widely varying conditions. Shift performance can be affected depending upon the operating environment, hydraulic fluid temperatures, and the like.
It is therefore desirable to provide a control arrangement for a transmission having a plurality of selectively operable hydraulic clutches wherein shift performance is monitored, and the sequential operation of the hydraulic clutches thereafter automatically adjusted for optimizing smooth and shock-free shifting.